Race/Color abbreviations on the 1920 US Census?

On the 1920 US Census, my great grandmother and her siblings have "MW" listed under the 'Race or Color' section. Her father's race clearly has 'B' (Black). I thought maybe it was for "Mulatto", but I thought Mulatto was just abbreviated to M. Does anyone have any clue... show more On the 1920 US Census, my great grandmother and her siblings have "MW" listed under the 'Race or Color' section.

Her father's race clearly has 'B' (Black). I thought maybe it was for "Mulatto", but I thought Mulatto was just abbreviated to M. Does anyone have any clue what the 'w' might stand for?
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